A solder-bearing lead is known in which the lead includes opposed resilient clamping fingers at one end of an elongated stem, with at least one of the fingers defining an electrical contact. The clamping fingers include opposed inner surfaces which define a gap for the reception of a rigid substrate circuit device therebetween, such that the inner surface on the electrical contact clamping finger engages a contact pad on the substrate circuit device. On an outer opposite surface of the contact clamping finger, the contact clamping finger carries a solder preform. The solder preform, upon being temporarily subjected to heat in a soldering operation, initially melts and flows over opposite sides of the contact finger onto the contact pad, and then resolidifies to bond the lead to the contact pad. Solder-bearing leads of this type are shown in U.S. Pat. No. 4,019,803 to M. S. Schell, and U.S. Pat. Nos. 4,120,558 and 4,203,648 to J. Seidler.
Solder-bearing leads as above described normally are fabricated in strip form in a progressive punch-and-die from a strip of phosphorous bronze base metal which has been provided with thin tin coatings on opposite sides thereof, to facilitate the subsequent making of electrical connections to the leads. During the lead fabrication process in the progressive punch-and-die, a continuous solder wire is attached to the contact fingers of the leads and subsequently clipped between the leads to form the solder preforms on the leads. Further, during the lead fabrication process the stems of the leads are formed integrally with an elongated continuous support rail which subsequently is clipped from the stems after the leads have been mounted on a substrate circuit device and soldered to respective contact pads on the device.
In fabricating the solder-bearing leads from a tin-coated phosphorous bronze strip as above-described, while portions of one of the tin coatings are wiped across a portion of the thickness of the base metal so as to coat the base metal, a major portion of the base metal thickness usually is not coated in this manner and is exposed to the atmosphere. When each lead then is mounted on a substrate circuit device the portion of the solder preform held by the contact finger of the lead is separated from the contact pad with which the contact finger is engaged, by phosphorous bronze base metal which, as a result of oxidation from exposure to the atmosphere, is not readily wetted by solder. Accordingly, instead of flowing down onto the contact pad in a soldering operation, the solder from the melted preform normally flows along the solder-wettable tin coating of the lead on the lead stem in a "wicking" action. Similarly, molten solder which does flow down to the contact pad then flows along the solder-wettable tin coating on the opposite side of the lead stem in a "wicking" action. Either flow is undesirable because the solder contaminates the tin coatings on the lead stem, sufficient solder may not be available on the contact pad to form a satisfactory soldered connection between the contact pad and the lead, soldering cycle time is increased, and the use of flux-bearing solder preforms, so as to eliminate undesirable spray fluxing of the assembled leads and substrate circuit devices, is not practical.
A proposed solution to the above-mentioned problems is disclosed in copending patent application Ser. No. 231,569, now U.S. Pat. No. 4,345,814, filed Feb. 4, 1981, in the names of E. A. Gutbier and P. J. Ouellette, assigned to the same assignee as the subject application, and entitled "Solder-Bearing Lead Having Solder Flow-Control Stop Means." In this regard, the E. A. Gutbier et al. application discloses a solder flow-control stop formed on the electrical contact finger of a solder-bearing lead closely adjacent a solder preform to preclude flow of molten solder from the solder preform along the contact finger to the lead stem, and to direct flow of the molten solder directly over the sides of the contact finger by gravity to the contact pad engaged by the finger during a soldering operation. Additional solder flow-control stops also may be formed on the lead to preclude flow of the molten solder along the lead stem during the soldering operation.
Monson et al. U.S. Pat. No. 4,302,067 discloses a solder-bearing lead in which portions of a solder preform carried on an outer surface of a lead contact finger directly engage a contact pad on a substrate circuit device to facilitate flow of molten solder from the preform over the contact pad in a soldering operation. In this regard, in the Monson et al. patent the contact finger is crimped about the center of the solder preform so as to deform the preform into an hourglass or "bowtie" configuration such that opposite ends of the preform engage the contact pad when the lead is mounted on the contact pad.
Copending patent application Ser. No. 402,631, filed on even date herewith in the name of R. D. Mackay and entitled "Solder-Bearing Lead Having A Solder Preform Engageable Directly With a Contact Pad," also discloses a solder-bearing lead in which a solder preform directly engages a contact pad on a substrate circuit device to facilitate flow of molten solder from the preform over the contact pad in a soldering operation. In this regard, in the R. D. Mackay application an inner surface of a contact finger of a lead is wrapped about the solder preform in a direction such that a surface portion of the preform projects from the contact finger for direct engagement with the contact pad when the lead is mounted on the contact pad. When the lead is temporarily subjected to heat in a soldering operation, the solder preform melts and flows directly over the contact pad and then resolidifies to form a soldered connection having an outer end portion of the contact finger embedded therein. In this regard, a purpose of this invention is to provide a solder bearing lead as disclosed in the Mackay application in which the contact clamping finger which carries the solder preform also directly engages the contact pad to which the lead is to be soldered, to produce an improved electrical and mechanical connection between the lead and the contact pad.